A36: Antidotes in Depth

Silibinin is the active ingredient in milk thistle, Silybum marianum, purported to prevent and treat amatoxin induced liver failure. Legalon SIL is a purified silibinin derivative available via an FDA approved open treatment as an investigational new drug (IND) for IV administration in patients suspected of ingesting amatoxin containing mushrooms.

Milk thistle has been used since the 16th century for the treatment of liver disease. Legalon SIL has been used in Europe since 1984.

Chemistry

Legalon SIL, silibinin-C-2′,3-dihydrogen succinate disodium salt (C₃₃H₂₈O₁₆Na₂) is a microcrystalline water soluble powder for injection manufactured by Rottapharm/Madaus, a German pharmaceutical company.⁴ Silibinin is a mixture of cis- and trans diastereomers of silibinin A and B extracted from milk thistle fruit and then esterified with succinic anhydride to form the water soluble salt.

Related Compounds

The extract from the seeds of the plant milk thistle (Silybum marianum) contains 65% to 80% silymarin. Silymarin contains many flavonoids ­including silybin A and silybin B, with a 1:1 ratio making up silibinin.¹³ Silymarin is poorly water soluble with a low bioavailability, but has substantial eneterohepatic cycling.

Mechanism of Action

Silibinin competitively inhibits the organic anion transporter (OATP1B3) that is responsible for the uptake and enterohepatic recycling of amatoxin. Early administration prevents initial uptake while later administration prevents enterohepatic recycling of the amatoxin.⁸ Other possible mechanisms of action include inhibition of hepatocyte TNF-α release, stimulation of mRNA protein synthesis, and antioxidant and antiinflammatory effects.

Pharmacokinetics and Pharmacodynamics

Legalon SIL is rapidly metabolized and eliminated from the blood. Precise details are not available.

There are no randomized clinical trials of Legalon SIL or silibinin in the treatment of amatoxin induced mushroom poisoning. Evidence for its use comes from observational studies in humans, a variety of animal studies with mixed results, and in vitro studies in cultured human hepatocytes.

Although in an experimental model of cultured canine hepatocytes silibinin was not able to reduce cytotoxicity, two experiments using cultured human hepatocytes demonstrated that the addition of silibinin was able to protect against lipid peroxidation and cytotoxicity, and also stimulate cell proliferation and attachment.^{5, 6, and 7}

One of the earliest animal studies was conducted in mice using sublethal doses of Amanita phalloides extract administered intraperitoneally (IP) every 24 hours for 3 doses.² Silymarin was administered intravenously (IV) at 16 and 24 hours, with the highest dose resulting in a 58% survival compared to 11% in the control group. When the experiment was repeated using a purified α-amatoxin, time was the most important factor in predicting survival. All of the mice treated with silymarin one hour before exposure survived, while none survived when treatment was started 8 hours after exposure.² A similar study in mice treated with IP α-amatoxin followed 4 hours later with IP silibinin dosed every 4 to 6 hours for 48 hours was not able to show a benefit in reducing the increase in LFTs or a difference in the liver histopathology.¹¹

A study conducted in canines utilized an oral sublethal dose of Amanita phalloides extract and IV silymarin administered at 5 and 24 hours postingestion. Silymarin prevented or reduced the elevation in LFTs and the fall in clotting factors and reduced the severity of liver failure.³ A similar protocol, but this time using silibinin instead of silymarin, demonstrated similar biochemical findings and a 100% survival.¹²

A 20 year retrospective review of European and American case reports of over 2000 hospitalized patients with amatoxin induced mushroom poisoning concluded that silibinin alone, or in combination, and N-acetylcysteine show the most promise as hepatoprotective therapies.¹ The authors acknowledged the difficulty in determining efficacy of various therapies given the lack of randomized controlled trials. A more recent review included the data from the previous authors and any subsequent hospitalized patients in the interim. A multidimensional multivariate statistical analysis of the data reached a similar conclusion.⁹

A retrospective case series of Amanita phalloides poisoning from Australia and New South Wales included 10 patients with probable poisoning, 9 of whom were treated with silibinin since use began in 2005.¹⁰ Four of these patients died. Mistaken identity of the mushrooms, particularly in immigrants, and a large number of ingested mushrooms seemed associated with a higher mortality. There was often a time delay to administration of Legalon SIL due to stocking and acquisition issues.

Since 1984, Legalon SIL has been administered to over 9000 patients with a good safety record.⁸ Mild flushing during intravenous infusion is commonly reported.

There are no data available with regard to pregnancy and lactation. Administration should be based on a risk-to-benefit analysis.⁴

Efficacy is thought to be related to prompt administration following the diagnosis of amatoxin induced mushroom poisoning. The loading dose of Legalon SIL is 5 mg/kg IV infused over one hour. This should be followed by a maintenance dose of 20 mg/kg IV as a continuous infusion over 24 hours and continued until an elevated PT/INR returns to normal and LFTs have decreased significantly.⁴ Each 350 mg vial of Legalon SIL should be reconstituted with 35 mL of D₅NS. The dose should be further diluted in D₅W. For example, after reconstitution, four vials (the 24 hour maintenance dose for a 70 kg person) can be put in 500 mL D₅W and infused IV over 24 hours.

Each vial of lyophilized Legalon SIL contains 350 mg of silibinin. Legalon SIL can be obtained by a physician via an FDA sanctioned, open treatment IND by calling the 24 hour, toll-free hotline (866-520-4412). A stockpile is kept in the United States, allowing same day shipment.

• Legalon SIL is a purified silibinin derivative for the prevention and treatment of amatoxin induced mushroom poisoning.

• Silibinin competitively inhibits the organic anion transporter (OATP1B3) that is responsible for the uptake and enterohepatic recycling of amatoxin.

• Silibinin should be administered as soon as possible following a presumed ingestion without waiting for mycologic confirmation.

• A 24 hour, toll-free hotline (866-520-4412) is available to obtain ­Legalon SIL.

References